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Investments (Tables)

12 Months Ended

Dec. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Investments

The following tables summarize our current investments as of the dates indicated:

	December 31, 2018
	Amortized				Gross Unrealized								Estimated
	Cost				Gains				Losses				Fair Value
	(In millions)
Corporate debt securities	$	1,131			$	—			$	8			$	1,123
U.S. Treasury notes	181				—				—				181
GSEs	164				—				1				163
Municipal securities	115				—				1				114
Asset-backed securities	83				—				1				82
Certificates of deposit	14				—				—				14
Other	4				—				—				4
Total current investments	$	1,692			$	—			$	11			$	1,681

	December 31, 2017
	Amortized Cost				Gross Unrealized								Estimated Fair Value
					Gains				Losses
	(In millions)
Corporate debt securities	$	1,591			$	1			$	4			$	1,588
U.S. Treasury notes	389				—				1				388
GSEs	255				—				2				253
Municipal securities	142				—				1				141
Asset-backed securities	117				—				—				117
Certificates of deposit	37				—				—				37
Subtotal - current investments	2,531				1				8				2,524
Corporate debt securities	101				—				—				101
U.S. Treasury notes	68				—				—				68
Subtotal - current restricted investments	169				—				—				169
	$	2,700			$	1			$	8			$	2,693

Contractual maturities of investments

The contractual maturities of our current investments as of December 31, 2018 are summarized below:

	Amortized Cost				Estimated Fair Value
	(In millions)
Due in one year or less	$	1,000			$	997
Due after one year through five years	692				684
	$	1,692			$	1,681

Available-for-sale investments

The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a continuous loss position for 12 months or more as of December 31, 2018.

	In a Continuous Loss Position for Less than 12 Months											In a Continuous Loss Position for 12 Months or More
	Estimated Fair Value				Unrealized Losses				Total Number of Positions			Estimated Fair Value				Unrealized Losses				Total Number of Positions
	(Dollars in millions)
Corporate debt securities	$	509			$	3			285			$	412			$	5			298
GSEs	—				—				—			127				1				76
Municipal securities	—				—				—			87				1				90
Asset backed securities	—				—				—			68				1				52
	$	509			$	3			285			$	694			$	8			516

The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a continuous loss position for 12 months or more as of December 31, 2017.

	In a Continuous Loss Position for Less than 12 Months											In a Continuous Loss Position for 12 Months or More
	Estimated Fair Value				Unrealized Losses				Total Number of Positions			Estimated Fair Value				Unrealized Losses				Total Number of Positions
	(Dollars in millions)
Corporate debt securities	$	1,297			$	3			561			$	94			$	1			69
U.S. Treasury notes	470				1				89			—				—				—
GSEs	173				1				69			95				1				47
Municipal securities	—				—				—			38				1				48
	$	1,940			$	5			719			$	227			$	3			164

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